A major problem with water pipes provided in ground is how to determine whether leakages from the pipes have occurred without having to dig ones way down to the pipes to control cracks or similar damages. One possible way to overcome this problem is to use the Doppler shift in reflected electromagnetic signals to determine whether there is a water flow outside of the pipes.
The Doppler shift of frequencies/wavelengths is a phenomenon that emerges when signals are reflected against a moving target. If the signals are emitted towards a target that is moving relative the position from which the signals was emitted, the wavelengths of the reflected signals will be altered relative the emitted signals. By utilising this phenomenon it becomes possible to determine whether a target is moving by means of emitting signals and detecting the reflected signals. If the wavelengths have become shorter compared to the wavelength of the emitted signals the target is moving towards the emitting source while longer wavelengths corresponds to the fact that the target is moving away from the signal source.
Presently Doppler radar is used to detect amount levels. The principle for this amount level detection resides on the fact that the water level is not constant, instead it will change over the course of long time periods, time periods of the order of minutes, because of water flow into and out of the system. By letting signals reflect on the water surface during a long time it is possible to add up the contributions to determine whether a Doppler shift has occurred. This is an expensive and time consuming method.
The present invention aims to provide a device and a method that overcomes the problems stated above by providing for a faster detection of leaks. The invention obtains this purpose by detecting pure fluctuations of the water surface. Such water surface fluctuations results, for example, because of a continuous change of the shape of the surface when water is flowing through a ground structure. In the case that the surface has a vertical movement this corresponds to the case that the reflector (the water surface) is moving relative the radar emitting source (see FIG. 1). Since the invention provides a signal processing system designed to detect particularly small Doppler shifts there is no need for the flow to be significant. Quite generally one can say that the magnitude of the water flow depends on a combination of, for example, the water amount, the depth and the velocity of the flow. Together these parameters give rise to velocity fluctuations in the vertical plane ranging from slow motions such a 0.01 m/s to very fast fluctuations of order 5-10 m/s.
A positive feature of a device and method according to the present invention is that it is possible to quickly detect the water leakage and take necessary actions before the leakage has lead to severe damages or to large water or gas losses.
The way that this object is achieved is through the use of a device according to claim 1 or a method according to claim 9.